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| United States Patent |
5,031,229
|
|
Chow
|
July 9, 1991
|
Deposition heaters
Abstract
A gaseous deposition source for providing a deposition material that
emanates from a crucible having mulitple thin film heating elements formed
thereon, with each adjacent pair being separated by an insulating layer
therebetween. A gaseous deposition source can have a crucible with a cover
thereon with one or more apertures therein and with thin film heating
elements on that cover about such apertures. A substrate heater may be
used formed of thin film heating elements provided on a base.
| Inventors:
|
Chow; Loren A. (2317 Byrnes Rd., Minnetonka, MN 55343)
|
| Appl. No.:
|
406785 |
| Filed:
|
September 13, 1989 |
| Current U.S. Class: |
392/389 |
| Intern'l Class: |
H05B 003/26 |
| Field of Search: |
219/271-276,436,438
392/388,389
|
References Cited
U.S. Patent Documents
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|
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|
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|
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|
| 4061800 | Dec., 1977 | Anderson | 219/271.
|
| 4146774 | Mar., 1979 | Fraas | 219/271.
|
| 4286545 | Sep., 1981 | Takagi et al. | 118/723.
|
| 4330932 | May., 1982 | Morris et al. | 29/579.
|
| 4396899 | Aug., 1983 | Ohno | 338/34.
|
| 4426569 | Jan., 1984 | Miller et al. | 219/272.
|
| 4447276 | May., 1984 | Davies et al. | 148/175.
|
| 4495155 | Jan., 1985 | Ricard et al. | 422/248.
|
| 4518846 | May., 1985 | Freeouf et al. | 219/271.
|
| 4534312 | Aug., 1985 | Shinya et al. | 188/666.
|
| 4543467 | Sep., 1985 | Eisele | 219/271.
|
| 4545339 | Oct., 1985 | Brooks et al. | 123/145.
|
| 4553022 | Nov., 1985 | Colombo | 219/275.
|
| 4560907 | Dec., 1985 | Tamura et al. | 315/111.
|
| 4587843 | May., 1986 | Tokura et al. | 73/204.
|
| 4662981 | May., 1987 | Fujiyasu et al. | 156/610.
|
| 4700660 | Oct., 1987 | Levchenko et al. | 118/726.
|
| 4726822 | Feb., 1988 | Cates et al. | 55/267.
|
| 4734563 | Mar., 1988 | Lloyd | 219/543.
|
| 4739657 | Apr., 1988 | Higashi et al. | 73/204.
|
| 4748315 | May., 1988 | Takahashi et al. | 219/275.
|
| 4748367 | May., 1988 | Bloch et al. | 310/343.
|
| 4782708 | Nov., 1988 | Harrington et al. | 73/861.
|
| 4812326 | Mar., 1989 | Tsukazaki et al. | 427/38.
|
Other References
Advertising Brochure: "Boralloy Pyrolytic Boron Nitride (PBN)/Pyrolytic
Graphite (PG) Resistance Heating Elements," Union Carbide Coatings Service
Corporation.
|
Primary Examiner: Walberg; Teresa J.
Attorney, Agent or Firm: Kinney & Lange
Claims
What is claimed is:
1. A material deposition source which can provide a gaseous material flow
through a plurality of apertures from a material provided therein in an
initial phase, said source comprising:
a crucible means having a containment shell means within which said
material in said initial phase can be selectively placed, said containment
shell means having a flow opening therein; and
an aperture cover means which can be fitted with respect to said crucible
means to be over said flow opening in said containment shell means, said
aperture cover means having an electrically insulating layer with a
plurality of apertures therethrough each of a selected orientation and
cross sectional area and further having formed thereon about and between
selected ones of said apertures a first thin film electrical heating
element to permit maintaining selected temperatures about those said
apertures.
2. The apparatus of claim 1 wherein said aperture cover means has a surface
of a convex shape through which said apertures extend.
3. The apparatus of claim 1 wherein said apertures extend through a surface
of said aperture cover means and intersects that surface at an angle other
than a right angle.
4. The apparatus of claim 1 wherein a second thin film electrical heating
element is provided on said aperture cover means about and between
selected ones of said apertures and over said first thin film electrical
heating element but separated therefrom by an insulating layer.
5. The apparatus of claim 1 wherein said aperture cover means has a thin
film thermocouple means formed thereon.
6. The apparatus of claim 1 wherein said aperture cover means has a
peninsular first tab portion extending away from that part thereof
supporting most of said first thin film electrical heating element but
with a portion of said first thin film electrical heating element
extending onto said first tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
7. The apparatus of claim 1 wherein said containment shell means in said
crucible means supports a second thin film electrical heating element
formed thereabout and situated so as to be able to heat interior portions
of said containment shell means and that said material placed therein.
8. The apparatus of claim 2 wherein a second thin film electrical heating
element is provided on said aperture cover means about and between
selected ones of said apertures and over said first thin film electrical
heating element but separated therefrom by an insulating layer.
9. The apparatus of claim 3 wherein a second thin film electrical heating
element is provided on said aperture cover means about and between
selected ones of said apertures and over said first thin film electrical
heating element but separated therefrom by an insulating layer.
10. The apparatus of claim 4 wherein said aperture cover means has a
peninsular first tab portion extending away from that part thereof
supporting most of said first and second thin film electrical heating
elements but with a portion of said first thin film electrical heating
element extending onto said first tab portion where it is adapted for
connection to electrical leads adapted for connection to a source of
electrical energy, and has a peninsular second tab portion extending away
from that part thereof supporting most of said first and second thin film
electrical heating elements but with a portion of said second thin film
electrical heating element extending onto said second tab portion where it
is adapted for connection to electrical leads adapted for connection to a
source of electrical energy.
11. The apparatus of claim 6 wherein said aperture cover means has a
peninsular second tab portion extending away from that part thereof
supporting most of said first thin film electrical heating element but
with a portion of said first thin film electrical heating element
extending onto said second tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
12. The apparatus of claim 7 wherein said containment shell means has a
peninsular first tab portion extending from that part thereof supporting
most of said second thin film electrical heating element but with there
being a portion of said second thin film electrical heating element
extending onto said first tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
13. The apparatus of claim 7 wherein said containment shell means has a
thin film thermocouple means formed thereon.
14. The apparatus of claim 7 wherein said containment shell means in said
crucible means supports a third thin film electrical heating element
formed thereabout over said second thin film heating element but separated
therefrom by an insulating layer.
15. The apparatus of claim 10 wherein said electrical leads are connected
to said first and second tab portions by a high temperature ceramic
compound.
16. The apparatus of claim 11 wherein said electrical leads are connected
to said first and second tab portions by a high temperature ceramic
compound.
17. The apparatus of claim 12 wherein said containment shell means has a
peninsular second tab portion extending away from that part thereof
supporting most of said second thin film electrical heating element but
with a portion of said second thin film electrical heating element
extending onto said second tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
18. The apparatus of claim 14 wherein said containment shell means has a
peninsular second tab portion extending away from that part thereof
supporting most of said second and third thin film electrical heating
elements but with a portion of said third thin film electrical heating
element extending onto said second tab portion where it is adapted for
connection to electrical leads adapted for connection to a source of
electrical energy.
19. The apparatus of claim 17 wherein said electrical leads are connected
to said first and second tab portions by a high temperature ceramic
compound.
20. The apparatus of claim 18 wherein said electrical leads are connected
to said first and second tab portions by a high temperature ceramic
compound.
21. A material deposition source which can provide a gaseous material flow
through an aperture from a material provided therein in an initial phase,
said source comprising:
a crucible means having a containment shell means within which said
material in said initial phase can be selectively placed, said containment
shell means having a flow opening therein; and
an aperture cover means which can be fitted with respect to said crucible
means to be over said flow opening in said containment shell means, said
aperture cover means having an aperture therethrough in a portion thereof
extending substantially into an interior region of said crucible means and
further having formed thereon about said aperture a first thin film
electrical heating element to permit maintaining selected temperatures
about said aperture.
22. The apparatus of claim 21 wherein a second thin film electrical heating
element is provided on said aperture cover means about said aperture and
over said first thin film electrical heating element but separated
therefrom by an insulating layer.
23. The apparatus of claim 21 wherein said aperture cover means has a
peninsular first tab portion extending away from that part thereof
supporting most of said first thin film electrical heating element but
with a portion of said first thin film electrical heating element
extending onto said first tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
24. The apparatus of claim 21 wherein said containment shell means in said
crucible means supports a second thin film electrical heating element
formed thereabout and situated so as to be able to heat interior portions
of said containment shell means and that said material placed therein.
25. The apparatus of claim 21 wherein said aperture cover means has a thin
film thermocouple means formed thereon.
26. The apparatus of claim 22 wherein said aperture cover means has a
peninsular first tab portion extending away from that part thereof
supporting most of said first and second thin film electrical heating
elements but with a portion of said first thin film electrical heating
element extending onto said first tab portion where it is adapted for
connection to electrical leads adapted for connection to a source of
electrical energy, and has a peninsular second tab portion extending away
from that part thereof supporting most of said first and second thin film
electrical heating elements but with a portion of said second thin film
electrical heating element extending onto said second tab portion where it
is adapted for connection to electrical leads adapted for connection to a
source of electrical energy.
27. The apparatus of claim 23 wherein said aperture cover means has a
peninsular second tab portion extending away from that part thereof
supporting most of said first thin film electrical heating element but
with a portion of said first thin film electrical heating element
extending onto said second tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
28. The apparatus of claim 24 wherein said containment shell means has a
peninsular first tab portion extending from that part thereof supporting
most of said second thin film electrical heating element but with there
being a portion of said second thin film electrical heating element
extending onto said first tab portion where it is adapted for connection
to electrical leads adapted for connection to a source of electrical
energy.
29. The apparatus of claim 24 wherein said containment shell means has a
thin film thermocouple means formed thereon.
30. The apparatus of claim 24 wherein said containment shell means in said
crucible means supports a third thin film electrical heating element
formed thereabout over said second thin film heating element but separated
therefrom by an insulating layer.
31. A material deposition source which can provide a gaseous material flow
from a material provided therein in an initial phase, said source
comprising:
a crucible means having a containment shell means within which said
material in said initial phase can be selectively placed, said containment
shell means having a flow opening therein;
a first thin film electrical heating element supported by and about said
containment shell means by adhering thereto through temperatures
sufficient to provide said gaseous material flow and situated so as to be
able to heat interior portions of said containment shell means and any of
that said material placed therein; and
a second thin film electrical heating element supported by and about said
containment shell means over said first thin film heating element but
separated therefrom by an insulating layer at least partly adherent to
said containment shell means, said first and second thin film electrical
heating elements being capable of heating said material to a temperature
sufficient to cause vaporization of said material.
32. The apparatus of claim 31 wherein said containment shell means has a
peninsular first tab portion extending from that part thereof supporting
most of said first and second thin film electrical heating elements but
with there being a portion of said first thin film electrical heating
element extending onto said first tab portion where it is adapted for
connection to a source of electrical leads adapted for connection to a
source of electrical energy.
33. The apparatus of claim 32 wherein said containment shell means has a
peninsular second tab portion extending away from that part thereof
supporting most of said first and second thin film electrical heating
elements but with a portion of said second thin film electrical heating
element extending onto said second tab portion where it is adapted for
connection to a source of electrical leads adapted for connection to a
source of electrical energy.
34. The apparatus of claim 32 wherein said electrical leads are connected
to said first tab portion by a high temperature ceramic compound.
35. The apparatus of claim 33 wherein said electrical leads are connected
to said first and second tab portions by a high temperature ceramic
compound.
Description
BACKGROUND OF THE INVENTION
The present invention relates to deposition sources and, more particularly,
to such sources for providing superior thin films of selected materials.
In recent years, thin film electronic and magnetic devices have become of
greater and greater commercial importance. As the need for precision in
providing such films has also increased, a number of methods have been
developed for more accurately providing such films. These include chemical
vapor deposition techniques, molecular beam epitaxy techniques, etc.
The success of such methods depends to a considerable extent on the
effusion cell which is the source of the atoms or molecules that are to be
deposited on a selected substrate. Such an effusion cell typically has a
crucible formed of high purity materials which are able to withstand high
temperatures while being maintained in a hard vacuum. A material from
which such a beam is to be formed is provided therein usually in a phase
other than gaseous. In common uses of such cells requiring the
beam-forming material to be converted to a gaseous state, crucible
structure materials are chosen that can operate at elevated temperatures
on the order of 1500.degree. C. and in a vacuum of typically 10.sup.-10
torr. Both outgassing from such materials, and the decomposition at such
temperatures and vacuums, must be avoided to avoid severely contaminating
the layers being deposited on the substrate. The presence and density of
the various kinds of atoms or molecules impinging on the substrate is
directly responsible for the composition of the layer being deposited.
The flux of the atomic or molecular beam impinging on the selected
substrate from the effusion cell is a direct function of the vapor
pressure of the beam material contained within the cell crucible. This
vapor pressure in turn depends on the temperature occurring in that
crucible, and depends rather strongly thereon as the flux has an
exponential-like relationship to temperature. Thus, fractions of a degree
of temperature can make significant changes in the beam flux. As a result,
the composition and thickness of layers to be deposited, if they are to be
reproducible, require that the crucible be accurately maintained at a
constant temperature.
However, there are a number of difficulties in maintaining such a constant
temperature in such a crucible and in avoiding contamination from crucible
structure outgassing or decomposition. Deposition sources today typically
have a serpentine conductive heating element positioned around the
crucible at a distance therefrom, and the heating of the crucible will be
mostly by radiation in these circumstances with little conduction. Such a
heating element is often constrained to have a shape that often does not
conform to the crucible shape thereby leading to low heating efficiency.
In addition, the heater temperature as a result is going to be
substantially higher than that to which the crucible is desired to be
raised, a situation which causes added outgassing from the heater element
and reduces it lifetime. The non-uniform spatial distribution of the
heating elements means that the crucible will have resulting hot and cold
zones making achieving of temperature uniformity difficult.
The design alternatives permitted for such a crucible are often limited by
considerations necessary in positioning such heating elements around the
crucible. The crucible usually has a relatively large opening at the
position the beam is to emerge therefrom which results in substantial
radiation loss through that opening thereby lowering crucible interior
temperatures nearby. This situation leads to uneven heating of the
beam-forming material contained within that crucible. Thus, there is
desired a deposition source which provides a crucible permitting more
uniform temperatures to be maintained therein. In addition, there is a
desire to provide a deposition source exhibiting reduced outgassing from
its components during use. Further, there is desire for a source which can
provide a material beam displaying good directivity. A further concern, in
those deposition processes in which a heated substrate is used, is the
uniformity of its heating and outgassing from the heater used therefor.
SUMMARY OF THE INVENTION
The present invention provides a gaseous deposition source for providing a
deposition material that emanates from a crucible having multiple thin
film heating elements thereon each pair of which is separated by an
insulating layer. The deposition source may have a crucible having a cover
thereon with one or more apertures therein and one or more heating
elements on that cover about such an aperture. These heating elements are
thin film heating elements, and further such elements can be provided
about the crucible as a source of heating for the deposition material
therein. Multiple ones of these heating elements can be provided in layers
for each with an insulating layer therebetween. Substrate heaters can also
be formed with such heating elements on a base.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a pictorial view of an embodiment of the present invention
having portions thereof separated from one another and a part of one of
these portions removed,
FIG. 2 shows a cross section view from FIG. 1 showing together the portions
shown separated there,
FIG. 3 shows a fragmentary cross section view of a portion of FIG. 2,
FIG. 4 shows a top view of a portion of the present invention,
FIG. 5 shows a cross section view of a portion of FIG. 4,
FIG. 6 shows an alternative cross section of a portion of FIG. 4, and
FIG. 7 shows a cross section view that is an alternative to that of FIG. 2
based on an alternative embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. shows a pictorial view of a high temperature crucible, 10, for use in
an effusion cell, and its cover 11, which is shown separated from crucible
10. Crucible 10 is shown to have a somewhat conical shape with a pair of
electrical terminal portions, 12 and 13, provided on the bottom thereof in
FIG. 1. In addition, crucible 10 is shown with its outer layers partially
cut away in FIG. 1 to reveal an inner layer and a thermocouple means, 14.
Similarly, cover 11 is shown with four terminal portions, 15, 16, 17 and
18, extending therefrom. In addition, cover 11 has a number of apertures,
19, extending therethrough.
A cross section view of the structure of FIG. 1 is shown in FIG. 2 with
cover 11 positioned on crucible 10. Crucible 10 is formed of an inner,
conical-like shape, containment shell or containment vessel, 20, formed of
high purity pyrolytic boron nitride with a wall thickness typically of 1.0
mm. This wall thickness is sufficient to provide adequate strength for
shell 20 at the elevated temperatures used in operation, as indicated
above. Shell 20 has a melt, 21, of a selected material provided therein in
a selected phase from which there is to be evaporation due to heating to
provide the basis for gaseous diffusion through cover 11. In some
circumstances, material 21 would be supplied from an appropriate source to
the interior of shell 20 already in gaseous form to be further heated
therein.
Formed on the outer side of shell 20 is a first layer heating element, 22,
spirally positioned on and about the outer surface of shell 20. A
peninsular tab portion of shell 20 extends outward from the bottom thereof
with a portion of heating element 22 provided thereon to form electrical
terminal portion 12.
Heating element 22 is formed of pyrolytic graphite which is applied to the
outer surface of shell 20 using a well known chemical vapor deposition
process, the deposition continuing to a typical thickness of from 1.0 to a
few mils to thereby set the thickness of heating element 22. After the
deposition of the pyrolytic graphite, the resulting graphite surface is
selectively masked through a well known process and the selected unmasked
portions thereof are etched away using again a well known process. The
result is to leave heater element 22 spirally positioned on and about the
outer surface of shell 20 and over the portion of crucible 10 involved in
terminal portion 12. Other heater position configurations could
alternatively be used.
The thickness chosen for heating element 22 is usually set by electrical
considerations given the limitations imposed on that element by
geometrical constraints and thermal mismatch considerations. The total
resistance of heating element 22 is chosen to be a value which will
accommodate the value of the voltage to be provided by a power supply
thereacross to ensure adequate heating with the element thickness set
accordingly.
Heating element 22 is encapsulated by an electrical insulating layer, 23,
formed of pyrolytic boron nitride provided again by a well known chemical
vapor deposition process. Insulating layer 23 is deposited to from 1.0 to
a few mils in thickness as sufficient to prevent any pinholes from
remaining therethrough which could lead to there being an electrical short
circuit between heating element 22 and the conductor to be provided on the
outer surface of insulating layer 23. Further, insulating layer 23 must be
sufficiently thick to prevent any voltage breakdown thereacross if heating
element 22 and such other outer surface conductor are operated at a
significant voltage differential.
A second layer heating element, 24, is then provided on insulating layer
23, once again using a well known chemical vapor deposition process.
Again, pyrolytic graphite is deposited as heater element 24, and this
deposition is then masked and etched to leave heater element 24 positioned
over at least the gaps between successive loops of heating element 22, the
heating element closest to shell 20. Thus, heating element 24 provides
heat in the gaps between the adjacent loops of heating element 22 to
thereby together provide a more uniform source of heat along the
conical-like walls of shell 20 to be applied to the interior thereof
including to material 21 contained in the interior of shell 20 in its
initial phase, as a melt and as a gas evaporated therefrom.
A small portion of heating element 24 is provided extending out onto a
peninsular tab portion formed outward from shell 20 to thereby form
terminal portion 13. External connections of wires can be made to this
terminal portion and terminal portion 12 by bonding one to the other
through use of high temperature ceramic bonding compounds such as known
bonding compounds having a combination of yttrium oxide, zirconia and
silicon nitride therein. Alternatively, high temperature fasteners can be
used to bond the external wires to these terminal portions by having such
fasteners inserted through the holes shown therein.
This uniformity of heating provided from a source closely adjacent to shell
20 allows heating elements 22 and 24 to operate at lower temperatures than
would otherwise be possible, and therefore provides longer life for these
heaters. In addition, there is less contamination of the layers deposited
on the selected substrate because of the resulting reduction in outgassing
from these heaters.
First layer heating element 22 and second layer heating element 24 can be
operated electrically in parallel with one another, or they can be
operated in series with one another. Alternatively, each may be operated
independently of the other. The use of multiple layers of heaters (more
than two could be provided) permits changing electrical parameters for the
combination of the heating elements involved so that a desired effective
resistance results even though there are space limitations and thermal
mismatch considerations which may limit how wide or thick a single layer
heating element can be.
The arrangement in FIGS. 1 and 2 is shown to be an electrical series
arrangement since only two terminal portions are shown to be available,
those being terminal portions 12 and 13. This has been made possible by
the use of a "feed through" formed by a portion of heating element 24
passing through insulating layer 23 to thereby be in electrical contact
with inner heating element 22. This can be seen in the upper right hand
side of FIG. 2, and in greater detail in FIG. 3 which is a fragmentary
view of a portion of FIG. 2 enlarged. This interconnection puts inner
heating element 22 and outer heating element 24 electrically in series
between terminal portions 12 and 13.
The temperature attained in operating crucible 10 by passing an appropriate
current through inner heating conductor 22 and outer heating conductor 24
can be accurately monitored through the presence of thin film thermocouple
14, shown on the left in FIG. 1 and shown on the left side of shell 20 in
FIG. 2. A pair of wires, each connected to an appropriate one of the pair
of overlapping thin films comprising thermocouple 14, are shown extending
therefrom in FIG. 2.
All of this structure on the outer side of shell 20 is then finally covered
by a protective layer, 25, of pyrolytic boron nitride, again deposited
using a well known chemical vapor deposition process to a thickness of 1.0
to a few mils. Protective layer 25 prevents outer conductor 24
therebeneath from adsorbing gaseous impurities when out in the open which
could later outgas at the crucible operating temperatures. Further, the
pyrolytic graphite in outer heater 24, in the absence of protective layer
25, may react with residual molecules occurring thereabout even after a
hard vacuum has been pulled therein. Note that all of the structure shown
on the outer walls of shell 20 is greatly exaggerated in thickness for
purposes of clarity.
This structure shown on the outer side of shell 20 in FIGS. 1 and 2 is
shown located there as a matter of choice. An entirely analogous structure
could alternatively be provided on the inner side of shell 20. Heating
conductors 22 and 24 can be lengthened sufficiently to reach terminal
portions extending from shell 20 such as 12 and 13 either over the upper
edge of, or through, shell 20.
Although crucible 10 is shown in FIGS. 1 and 2 to be conical-like in shape,
other shapes are possible and even made more feasible by the use of thin
film heaters deposited directly on the inner or outer walls of the
containment vessel. That containment vessel could, for instance, be caused
to constrict flow before reaching the effusion opening by being formed
into a vase-like shape through having a narrow, but open, neck near that
effusion opening. The vessel from there flares out to from the effusion
opening to thereby reduce radiation losses through that output opening.
Temperature uniformity for such a structure is enhanced by having inner
heating element 22 or outer heating element 24 provided near such a
"necked down" portion of crucible 10. Further, the resulting flared
section past this narrow opening could also have heating elements formed
thereon which would help to form the molecular beam based on the gaseous
material effusing from the constricted portion of the crucible.
A more direct means for accomplishing such results is to use perforated
cover 11 shown in FIG. 1 and in more detail in FIGS. 2 and 4. FIG. 4 shows
the top view of cover 11 having apertures 19 extending therethrough.
Formed about apertures 19 on a substrate, 20', are two further thin film
heating elements, including an inner heating element, 22', shown in dashed
lines, and an outer heating element, 24', also shown in dashed lines
though dashed differently. These two heating elements are again separated
by an insulating layer, 23', of pyrolytic boron nitride with heating
element 22' being formed directly on the pyrolytic boron nitride substrate
used for cover 11. Again, a pyrolytic boron nitride outer protective
layer, 25', covers the structure shown therebelow in FIGS. 2 and 4.
Provision of this structure on substrate 20' used for cover 11 is done in
the same manner as the provision of the structure on the outer side of
shell 20 in forming crucible 10. Apertures 19 can be drilled either before
or after such processing has been completed.
As can be seen in FIGS. 2 and 4, inner heating element 22' and outer
heating element 24' each have both ends thereof coming onto a
corresponding pair of terminal portions based on peninsular tab portions
extending downward and outward from the substrate on which cover 11 is
based. Outer heating element 24' has its ends terminating on terminal
portions 15 and 18, and inner heating element 22' has its ends terminating
on terminal portions 16 and 17. External connections of wires can be made
to these terminal portions through use of high temperature ceramic
compounds to bond one to the other. Alternatively, high temperature
fasteners can be used to bond the external wires to these terminal
portions by having such fasteners inserted through the holes shown
therein. A further thin film thermocouple can be provided in cover 11 of
the same nature as thermocouple 14 described above.
The use of cover 11 reduces radiation loss from the opening of crucible 10
considerably to thereby keep the beam-forming material inside crucible 10
much closer to being at a constant temperature everywhere. Placing such a
cover over the effusion opening or exit for the gaseous phase of this
material in crucible 10 allows a considerably higher vapor pressure to be
generated therein which, with a relatively uniform temperature, permits
constant and reproducible molecular beam fluxes to be provided with more
uniform deposition results.
The provision of cover 11 also acts as a stop to prevent "spitting" or
"spilling" of the beam-forming material. Such actions occur in the
beam-forming material because of trapped gases therein or surface volatile
compounds, or even because of the material "creeping" along the crucible
wall due to surface tension, and often lead to defects in the thin films
being deposited on a selected substrate. Thus, provision of cover 11
eliminates such undesirable results.
Inner heating element 22' and outer heating element 24' provided
independently on cover 11 permit cover 11 to be operated at different, and
particularly, at higher temperatures than crucible 10. Such higher
temperatures in cover 11 prevent condensation of the beam-forming material
effusing at apertures 19 which can easily cause undesirable changes in the
beam flux. Also, the covering of heating elements 22' and 24' by
protective layer 25' keeps them from being exposed directly to the
substrate on which thin films are being deposited. This avoids the
incorporation into the films being deposited of contaminants arising from
the heating of these heating elements.
FIGS. 5 and 6 show two examples of the shaping of either cover 11 or
apertures 19 provided therethrough to thereby control the spatial
distribution of the material beam. FIG. 5 shows apertures 19 being formed
in a flat version of cover plate 11. However, apertures 19 in that version
intersect the surfaces of that cover at an angle with respect to a
perpendicular to such surfaces so that the beam-forming material escaping
through apertures 19 tends to have dominant velocity components such as
cause the beam to converge. On the other hand, cover 11 can just as well
have a convex shape with respect to the inside of crucible 10, but with
apertures 19 then paralleling the corresponding local perpendicular to the
surface of cover 11. Again, the beam will tend to converge at some point
beyond the effusion cell.
FIG. 7 shows an alternative cover plate, 11', formed on a substrate, 20'',
having an inner heating element, 22'', and an outer heating element, 24'',
separated by an insulating layer, 23'', provided thereon, and covered by a
protective layer, 25''. Heating elements 22'' and 24'' are not shown
mostly perpendicular with respect to each other as in FIGS. 2 and 4 but,
instead, are mostly parallel to each other with one being mostly
positioned in the gaps between adjacent portions of the other. In FIG. 7,
inner heating element 22''0 has an end thereof shown terminating on a
terminal portion, 16'', and outer heating element 24'' has an end thereof
shown terminating on a terminal portion, 17''. These structural portions
are each formed as the corresponding structural portions shown in FIGS. 1
and 2.
A single aperture, 19', occurs through cover 11' in a central portion
thereof formed as a truncated cone which extends inward into the interior
of crucible 10. This arrangement for aperture 19' can also provide
material beam directivity and good flow volume, and can in some
circumstances provide more uniform heating at this aperture because of its
position in the interior of crucible 10.
If apertures 19 are omitted, cover 11 can alteratively serve as a flat
heater for heating substrates upon which material depositions are to be
made. The multiple heating elements 22' and 24' will provide very good
temperature uniformity across such a structure. Further, the covering of
such heating elements in the resulting structure will prevent outgassing
therefrom.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize that
changes may be made in form and detail without departing from the spirit
and scope of the invention.
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